1. Field of Invention
The invention relates to a sewage treatment system and more particularly to a sewage treatment system characterized by removal of solid components in sewage by high-rate filtration through floating filter media, removal of solid components adhered to the filter, and biological treatment of sewage after the solid components have been removed.
2. Description of the Related Art
Generally, the conventional sewage treatment process cleans the sewage and releases the effluent into public waters such as river and lake by a two part process. The first part includes natural sedimentation of solid components from the sewage in a primary sedimentation tank, and the second part includes biological treatment by microbes in an activated sludge treatment system. Activated sludge that has been treated biologically settles as sediment in a final sedimentation tank. Then the supernatant liquid of the sewage is released and a part of the activated sludge is taken out as surplus sludge. The remaining part of the activated sludge is sent to the activated sludge treatment system. Discharging the effluent only after the sewage has been treated to conform to the water quality standards for public waters, improves the quality of public waters.
However, recently, as urbanization has increased and living standards have risen, the concentration of water contaminants, the amount of sewage, and the number of sources of sewage have increased.
The conventional primary sedimentation tank activated sludge treatment system cannot meet the treatment needs because the solid component removal capacity by natural sedimentation in the primary sedimentation tank lacks sufficient efficiency. Therefore, a filtration system using a high-rate filtration process has been developed to remove solid components quickly. According to the high-rate filtration system, a layer of floating filter media, that is made of granular filter media having a specific gravity smaller than that of the sewage, is provided for filtering the sewage. When the filtering performance declines by solid components in the sewage saturating the filter medium, the filter is cleaned to reactivate the filtering capability. Therefore, the filtration rate is higher than that of the primary sedimentation tank. Such high-rate filtration systems are disclosed in Japanese Patent Application Laid-open No. 51-85257, Japanese Patent Application Laid-open No. 60-193508, and Japanese Utility Model Application Laid-open No. 2-12405.
The conventional filtration system using a high-rate filtration process has a higher filtration rate than the primary sedimentation tank, and therefore a smaller area is required for the treatment system. However, the filter medium may become blocked quickly, shortening the break through point, thereby losing filtration performance. Therefore, the filter medium must be cleaned frequently. Further, another disadvantage is that large effluent volumes of filter backwash water are generated. A further disadvantage is that the flow resistance reduces the filtration rate.
In a conventional combined sewage treatment system, when an extraordinary amount of rain water delivers an amount of sewage inflow beyond the capacity of the activated sludge treatment system, the excess sewage must be released to public waters merely after removing the solid component. Therefore, the solid components in the sewage must be removed to the greatest extent possible before that release. However, in the conventional sewage treatment system, the sewage is released into public waters without sufficient solid component removal.
Because conventional systems lack sufficient capacity for peak inflow, the quality of public waters deteriorates. The conventional system lacks technology for handling sudden increases of inflow.
Under these circumstances, it is desirable to develop a sewage treatment system that:
(1) provides efficient filtration of solid components in sewage, provides efficient cleaning of filter medium for reuse, and is compact; and PA1 (2) provides high-rate filtration and biological treatment with adequate capacity for peak inflow of sewage.